The invention relates to a device for doping, deposition or oxidation of semiconductor material at low pressure in a process tube, which is provided with a tube closure as well as with devices for supplying and discharging process gases and for generating a negative pressure in the process tube.
In comparison to diffusion at atmospheric pressure, diffusion at low pressure makes it possible, as well known, to decrease the spacing between the wafers and, thus, to load the process tube with a higher number of silicon disks while simultaneously retaining or improving the homogeneity of the doping operation. The prerequisite is that it must be possible to evacuate the process tube or the furnace, which has to be vacuum tight, so that an adequately low processing pressure is reached. For example, a processing pressure of about 200 mbar may be regarded as an adequately low processing pressure.
Furthermore, the output and reaction products may not come into contact with materials that would be attacked thereby; and these products may not accumulate in this reaction tube/furnace.
The past prior art devices (for example, EP 1 393 351 A1) for phosphorus doping at low pressure with phosphorychloride as the dopant exhibit considerable problems. For example, a condensation of phosphorus oxide occurs on the surfaces and, in particular, in the region of the tube closure as well as on the end of the process tube, to which a pump is connected, and also in the waste gas zone between the process tube and the pump as well as in the pump itself. The reason lies in the fact that the temperature in these regions is significantly lower than the processing temperature.
The contact with water, in particular of atmospheric humidity following aeration of the device with pure nitrogen and pure oxygen upon opening the tube closure causes the phosphorus oxide to convert into phosphoric acid. The fatal consequences are corrosion of the metallic components of the device, such as the tube closure, and the subsequent contamination of the process tube with the corrosion products and the contamination of the products, which are processed in the device, for example due to the iron contamination in silicon.
Furthermore, there is the risk that the phosphoric acid will escape from the process tube or that the reaction products may accumulate in the process tube and the components of the device that are connected to said process tube. The components that are connected to the process tube may coalesce; and there is the risk of decomposition of the process tube and the components that are connected to said process tube. Finally the accumulation may unfold an undesired doping effect.
Moreover, the reaction products, like chlorine, hydrochloric acid, phosphorus oxide and phosphoric acid, may cause corrosion, including quartz corrosion.
In a device of the “cantilever” construction the paddle stays in the process tube during the process, is consequently heated to the processing temperature and is then removed again at a high temperature after the end of the process. For example, in the cantilever design the rear end of the paddle exhibits a cylinder, the surface of which is enveloped by a sealing ring.
The invention is based on the problem of providing a device for doping, deposition and oxidation of semiconductor material at low pressure in a process tube. With this device the aforementioned drawbacks are to be avoided.